Temporal changes in Hg, Pb, Cd and Zn environmental concentrations in the southern Baltic Sea sediments dated with 210Pb method

ORIGINAL

RESEARCH

ARTICLE

Temporal

changes

in

Hg,

Pb,

Cd

and

Zn

environmental

concentrations

in

the

southern

Baltic

Sea

sediments

dated

with

210

Pb

method

Tamara

Zalewska

*

,

Jerzy

Woro

ń

,

Beata

Danowska

,

Maria

Supli

ń

ska

a

InstituteofMeteorologyandWaterManagement—NationalResearchInstitute,MaritimeBranch,Gdynia,Poland b_{CentralLaboratoryforRadiologicalProtection,Warsaw,Poland}

Received26June2014;accepted27June2014

Availableonline23October2014

— KEYWORDS 210 Pbdating; Heavymetals depositionhistory; BalticSea; Environmentalstatus assessment

Summary Thearticlepresentsdataonheavymetal—Hg,Pb,CdandZn—distributioninthe

layersofmarinesedimentsfromtheoff-shoreareasofthesouthernBalticSea:GdańskDeep,SE

Gotland BasinandBornholm Deep.Depthprofilesofmetalconcentrationswereconvertedto

time-basedprofilesusingthe210Pbdatingmethodandverifiedby137Csdistributioninthevertical

pro_file. Thelinearsedimentation ratesin theGda_ńskDeepand SEGotlandBasin aresimilar,

0.18cmyr1and0.14cmyr1,respectively,whiletheregionoftheBornholmDeepis

charac-terizedbyagreatersedimentationrate:0.31cmyr1_{.Regardinganthropogenicpressure,Gdańsk}

Deepreceivesthelargestshareamongtheanalyzedregions.Themaximalmetalconcentrations

detectedin thisareawereZn—230mgkg1_,Pb—77mgkg1_{,Cd—2.04mgkg}1_andHg—

0.27mgkg1.TheleastimpactofanthropogenicpressurewasnoticeableinSEGotlandBasin.The

combination ofsediment datingwith theanalysisof verticaldistribution of heavymetalsin

sedimentsbenefitedinthedeterminationoftargetmetalconcentrationsusedinenvironmental

statusassessments.Referencevaluesofheavymetalconcentrationsinmarinesedimentswere

determinedas:Zn_—110mgkg1_,Pb—30mgkg1_{,Cd—0.3mgkg}1_{andHg—0.05mgkg}1_from

theperiodofminoranthropogenicpressure.Basingonthedeterminedindices:enrichmentfactor

PeerreviewundertheresponsibilityofInstituteofOceanologyofthePolishAcademyofSciences.

* Correspondingauthorat:InstituteofMeteorologyandWaterManagement_—NationalResearchInstitute,MaritimeBranch,Waszyngtona42, 81-342Gdynia,Poland.Tel.:+48586288266;fax:+48586288163.

E-mailaddress:[email protected](T.Zalewska).

Availableonlineatwww.sciencedirect.com

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1.

Introduction

Intensificationofhumanactivitiesregardingnew technolo-gies,especiallyinventingnewsubstances,progressin med-icineandpharmaceuticalindustryandtheextensionofneeds in progressing civilization in general,results in increasing anthropogenic pressure on the natural environment. The release of large amounts of chemical substances to the environment poses currently one of the serious problems as neither their effects nor their distribution among the environment components is well recognized. Therefore, the assessment of the environmental status became the keyissueatpresentinordertosupportappropriatedecisions onmeasuresaimingatreductionofthepressuresand restora-tionof theundisturbedfunctioning of theecosystem.The HELCOMBalticSeaActionPlan(HELCOM,2007)isanexample of such a voluntary initiativeof countries wishingto have back a healthy sea, and the Water Framework Directive (WFD) (Anon., 2000) and the Marine Strategy Framework Directive(MSFD)(Anon., 2008)are theexamplesof strong legal actions that bind countries to undertake measures aimingatprotection ofthemarineenvironment. Nonethe-less,thefirststageinanycounter-measureistheappropriate assessmentofthecurrentenvironmentalstatusand compar-isonwith certain reference statusassumedas the desired one.Inthecaseofpollutants—hazardoussubstances—the parametersusedtodefinethegood/desiredenvironmental statusare usually certain targetvalues of concentrations. Itisassumedthatconcentrationslowerthanthetargetare innocuous.Itisthenofgreatimportancetodeterminethe environmental target for any harmful substance. One of the possible ways is the determination of contaminant (e.g. heavy metal) concentrations related to moderate anthropogenic impact thatwouldnextallow to determine thereferenceconditions/backgroundvalues.

Itwaspointedoutthatthedeterminationofbackground levelsoftheanalyzedheavymetalsisveryimportant regard-ing the choice of the appropriate assessment metrics; hence, it is the key issue in the finalassessment result, e.g.:geoaccumulationindex—Igeoorenrichmentfactor— EF(CarvalhoGomesetal.,2009;Pempkowiak,1991; Pemp-kowiaketal.,1998;Rubioetal.,2000;Zahraetal.,2014). Thedeterminationofreferencevaluesforheavymetalsin sedimentsoftheassessedareaisanoptimalsolutioninthis case;however,relyingsolelyongeochemistry-based inves-tigationmightnotbesufficient,andsedimentdatingseems tosupplyunequivocalinformationontheperiodwhichhasto beconsidered fortheidentificationof background values (Álvarez-Iglesiasetal.,2007;CarvalhoGomesetal.,2009; Díaz-Asencio et al., 2009; Ruiz-Fernández et al., 2004; Sanchez-CabezaandDruffel,2009).Sedimentsarethesole environmentalelementsthatreflectthechangesongoingin amarineenvironmentinasystematicandnearlypermanent

way. This feature is of particular interest regarding the distributionandaccumulationofcontaminantswhose con-centrationsare subjecttointense variabilityin seawater andmarine organisms.Thechanges observedin pollution ofthemarineenvironmentbecomepermanentlypreserved in the sediments. The mechanism directly responsible is thefactthat contaminants,includingheavymetals,show a significant affinity to suspended organic matter (Pempkowiak et al., 1999; Roussiez et al., 2005; Rubio et al., 2000), and having been adsorbed and/or bio-accumulated in organicmatter, they are amassed in the sedimentsduetoverticaltransportandsedimentation pro-cesses(Álvarez-Iglesiasetal.,2007;CarvalhoGomesetal., 2009; Díaz-Asencio et al., 2009; Ruiz-Fernández et al., 2004).Therefore,sedimentsmayactasarecordofhuman impactinareaswheretheformationofconsecutivelayers proceedsinanunperturbedway.Combininginformationon contaminant changes in sediments with sediment dating basedontheanalysisoftheleadisotope—210_Pbpresents uswithversatileapplicationprospects.

Since1963,whenGoldberg(1963)presentedhismethod of sediment dating by 210Pb, a number of authors have further developed the method and its application as a geochronological tool in sediment studies in various environmental systems (lakes, estuaries, marine areas) (ApplebyandOldfield, 1992;Appleby,1997;Díaz-Asencio etal.,2009;Matsumoto,1987;Mulsowetal.,2009;Pfitzner etal., 2004; Suplińska and Pietrzak-Flis, 2008; Zaborska etal.,2007,2014;Zajączkowskietal.,2004).Themethod ofsedimentdatingbasedonananalysisof210Pb concentra-tionchangesmakesitpossibletocharacterizethescaleof 100—150yearsback,i.e.theperiodofintensive industria-lizationandincreaseofhumanactivitiesinallaspectsof existence. Sediment dating allows the identification of potentialpollutionsourcesandtheexaminationof contam-inationchangesrelatedtotransport(Álvarez-Iglesiasetal., 2007;Ayrault etal., 2012; CarvalhoGomesetal., 2009; Díaz-Asencioetal.,2009;Lietal.,2012;Ruiz-Fernández etal.,2004).

The presented studyfocused on the applicationof the datingmethod,basedontheverticaldistributionof210Pbin marinesediments, to the determination of sedimentation ratesandthedatingofserialsedimentlayersintheareasof thesouthernBalticSeacharacterizedbyundisturbed sedi-mentation. By combining this information with results on heavymetalHg,Cd,PbandZndistributioninthesedimentsit was possibleto establishenvironmental target concentra-tionsof heavy metals: Hg, Pb, Cd; the priority hazardous substances taken into account in environmental status assessment.Basingon thedeterminedindices: enrichment factor (EF), geoaccumulation indicator (Igeo) and contam-inationfactor (CF) the statusof marineenvironment was assessedregardingthepollutionwithheavymetals.

(EF), geoaccumulation indicator (Igeo) and contamination factor (CF) the status of marine

environmentwasassessedregardingthepollutionwithheavymetals.

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2.

Material

and

methods

2.1. Studyarea

The areasselected for thestudy: BornholmBasin, Gdańsk BasinandSEGotlandBasin(Fig.1),arecharacterizedbythe occurrenceofsilt-claysediments,i.e.theBalticolive-gray mud,containingmainlyfractions finerthan0.063mm.The bottomsof these areasare frequentedby the occurrence of strong oxygen deficit and anaerobic conditions, and laminated deposits without bioturbation structures reflect theannualsedimentaryrhythmicity.Theaccumulationrate ofthesilty-claycanvaryinarelativelywiderangefrom0.5to 2mmyr1_{(Uścinowicz,2011).}

2.2. Sampling

Sedimentsampleswerecollectedatthreesamplingstations locatedinthesouthernBalticSea:P5of87mdepthinthe BornholmDeep,P140of89mdepthintheGotlandBasinand P1ca.107mdepthintheGdańskDeep(Fig.1).Thesamples weretakenwithaNiemistöcorerwithinnerdiameterof5cm onboardr/vBalticaduringroutinemonitoringcruises.

For the purpose of heavy metal determination, three parallelcores werecollected,eachthen dividedinto2cm slices down to 10cm depth, anddeeper 2cmslices were selectedat every5cmlengthofthe core.Eventually,the threeparallelcoresweredividedintothefollowingsamples: 0—2,2—4,4—6,6—8,8—10,15—17,22—24,29—31and36— 38cm.Theselectionofsedimentlayersandthenumberof samplessufficienttoassessheavymetaldistributioninthe sedimentcorewasassumedbasedonmultiyearexperience gained in monitoringactivities of theBaltic Sea. The wet

sedimentsampleswerepreservedbydeep-freezingonboard the vessel and were then freeze-dried, homogenized and storedforfurtheranalysesonarrivalatthelandlaboratory. For the purpose of sediment age determination, six parallel sediment cores were taken at each station. The coresweredividedinto1cmwideslicesdownto5cmdepth anddeeperintoslicesof2cmwidth.Thisyieldedthe follow-ingsedimentlayers:0—1,1—2,2—3,3—4,4—5,5—7,7—9, 9—11,11—13,13—15,15—17and17—19cm.The correspond-ingslices/layersfromthesixparallelcoresatthesampling stationwereintegratedtoproduceasingleanalytical sam-ple.Thesesamples wereinitiallydeep-frozen onboardthe shipandfreeze-driedandhomogenizedinthelandlaboratory priortotheexactanalysis.

2.3. Modelsfor 210_{Pb sedimentdating}

210

Pb identified in sediment samples originates from two sources. A certain fraction is the result of radium(226Ra) radioactive decay and this is called supported 210Pb (210_Pb

supp); its activity along the verticalsediment profile practically does not change. The other source of 210Pb depositedinmarinesedimentsisatmosphericfall-out. The activityof210Pbunsupportedorexcess(210Pbex),originating from theatmosphericdeposition,decreaseswith the sedi-mentdepth.Thisactivityformsthebasisinthe determina-tionofratesofsedimentaccumulation:massaccumulation rate(MAR)andlinearaccumulationrate(LAR)andintheage determination of particular sediment layers. The 210Pbex activityconcentrationisdeterminedfromthetotalactivity ofthisisotope(210Pbtot)intheanalyzedlayerbysubtracting theactivityofoneoftheproductsof226_Radecay,e.g.214_Bior 214

Pb.Inthepresentstudy,determinationsofsedimentation

ratesandsedimentagealongtheverticalprofilesweredone usingtwomodels(ApplebyandOldfield,1992;Appleby,1997; Boeretal.,2006;CarvalhoGomesetal.,2009;Díaz-Asencio et al., 2009; Robbins, 1978; Szmytkiewicz and Zalewska, 2014).Thefirstmodel—theConstantRateofSupply(RSC) model—isbasedontheassumptionthatthesupplyof210_Pb totheseasurfaceisconstant,whilethesedimentationrate mightvary.Thismodelseemstoapplytomostsedimentary systems where sediment supply may vary in response to climatic or anthropogenic changes. The second model — the Constant Flux Constant Sedimentation Rate (CF:CS) model — assumesa constant dry-mass sedimentation rate (SzmytkiewiczandZalewska,2014).

Inordertoverifytheresultsofagedeterminationbythe 210_{Pbmethoditisnecessarytoapplyanadditionaltagwhose} concentration changes in the marine environment can be easilydocumentedtospecificevents.InthecaseoftheBaltic Sea,themostobvioustagisthetotallyanthropogenicisotope ofcesium—137Cs.Intheverificationoftheagedetermination method based on 137_{Cs it is assumed that the described} historicalevents(e.g.testingofnuclearweaponsperformed since1945with maximumdepositionrecorded in1963and theaccidentintheChernobylpowerplantin1986)shouldbe well marked, as an increase, in the curve of the isotope changesalongthesedimentcore.Simultaneously,theresults have to beinterpretedcautiously,takingintoaccount the complexityandlargenumberofprocessesaffectingthefinal result—thepresentationof137Csdistributioninthesediment verticalprofile. Therefore the isotope could be usefulfor verifying sedimentchronologywhen post-depositional pro-cesses are not affecting this radionuclide (Díaz-Asencio etal.,2009).

2.4. Analysisofgammaemitters 210

Pb, 226Ra and 137Cs contents in marine sediments were analyzed by high-resolutiongamma spectrometryusing an HPGe detector with a relative efficiency of 40%, and a resolution of 1.8keV for peak of 1332keV of 60_{Co. The} detectorwas coupledwith an8192-channelcomputer ana-lyzer (GENIE 2000). The samples were placed in plastic containersofgeometryidenticalto thoseused for calibra-tion.Afterreachingequilibriumbetween226Raandits daugh-ter nuclides (214_Bi, 214_{Pb) the samples were ready for}

measurements.Timeofmeasurementswas80,000sforeach sample. 210Pb was determined by gamma emission at 46.5keV,226Rawasdeterminedbytheemissionofits daugh-ternuclides214_Pband214_{Biat352keVand609keV} respec-tivelyand137Cswasmeasuredviaitsemissionat661.6keV. Thereliabilityandaccuracyoftheappliedmethodwere verifiedbythemeasurementofcertifiedsedimentmaterial IAEA-300(Table1).

2.5. Analysis ofheavymetals

Mercury content in sediments was determined using cold vaporatomicabsorptionspectrometryinanAMA254mercury analyzer.Inthismethod,asample(ca.100mg)isplacedin theburningchamberoftheanalyzer,whereitisdriedand burnedinoxygenflame at 6008C.The releasedmercuryis collectedinagoldamalgamcatalyst.Havingcompletedthe sample decomposition, the temperature is stabilized at 1208C and mercury content is measured with a detection limitof0.05ng.

Cadmium,lead,zincandaluminumconcentrations were measured in the sediments' mineralization, obtained by treatingthesedimentsamples(ca.1.5g)withconcentrated acids HNO3 and HF. The mineralization was carried out in teflonvesselsatelevatedtemperature.Theconcentrations ofmetalsweremeasuredusing atomicabsorption spectro-metry(AAS);cadmium—inaPerkin-Elmer4100spectrometer withHGA700graphitefurnace,andlead,zincandaluminum weremeasuredinaShimadzuAA-6601Fflameatomic absorp-tionspectrometer.

The accuracy and precision of measurements were controlled using a certified reference material (Table 1), analyzedparalleltothesedimentsamples.

3.

Results

and

discussion

3.1. Sediment geochronology 210_Pb

ex activity along the sediment cores from the three sedimentationbasinsofthesouthernBalticSeawas calcu-latedasthedifferencebetweenthe210_Pb

totactivityandthe 210

Pbsuppactivity;thelatterbeingdeterminedbymeasuring theactivityoftheradioactivedecayproducts(214Bi,214Pb) of 226_{Ra which remain in radioactive equilibrium with}

Table1 Resultsofcertifiedreferencematerialanalyses.

Certi_fiedreferencematerial Analyzedelement Certi_fiedvalue Con_fidenceinterval Measuredvalue

IAEA_—300,Radionuclides

intheBalticSea

137_{Cs 1056.6Bqkg}1 onreference date:1.01.1993 1046_—1080 106524Bqkg1 onreferencedate: 1.01.1993 210 Pb 360Bqkg1on referencedate: 1.01.1993 339—395 35040Bqkg1 onreferencedate: 1.01.1993 QTM045MS Hg 0.103mgkg1 _{— 0.1030.004mgkg}1 QTM080MS Cd 0.490mgkg1 _{— 0.4630.023mgkg}1 Pb 48.80mgkg1 — 47.291.64mgkg1 Zn 148.99mgkg1 _{— 145.933.51mgkg}1

210

Pbsupp.Inallthreebasins,activityconcentrationsofboth 210

Pbtot and210Pbsupp declineexponentially with the sedi-mentdepth,thusprovidingabasisfortheapplicationofboth datingmodelsCF:CS and CRS(Boer etal., 2006;Carvalho Gomes et al., 2009; Díaz-Asencio et al., 2009). Similar distribution was observedin Gdańsk Deep by Pempkowiak (1991). 214Bi activities along the core profiles changed in relativelynarrowranges.Themaximaldifferencesinagiven basinwereupto10Bqkg1,withthemeanatthelevelof39 3Bqkg1intheBornholmDeep,353Bqkg1intheSE GotlandBasinand457Bqkg1intheGdańskDeep.

Themaximalactivityconcentrationsof210Pbexwerefound in the surface sediment layer in the Gdańsk Deep (420Bqkg1)and decreased rapidly to 5Bqkg1 at 21cm depth of the sediment. In the SE Gotland Basin activity concentrations varied from 4Bqkg1 in the deepest sedimentlayers to 242Bqkg1 _{in the surfacelayer, while} theyoungestsediments oftheBornholmDeep showedthe leastactivityof210Pbex(151Bqkg

1 ).

The age of individual sediment layers was determined usingtheCRSmodelapplyingthecumulativedepthinsteadof therealdepthtoeliminatethesedimentcompactioneffect andrelatedvariablecontentofinterstitialwater(Fig.2).The obtained statistically significant correlations between the ageofsedimentlayerandcumulativedepthweredescribed with 2nd degree polynomial function in all three studied sedimentationbasins. Thecorrelation coefficientsreached 0.980,0.999and0.997 inthe SEGotlandBasin, Bornholm DeepandtheGdańskDeep,respectively,atconfidencelimit ofp=0.0000(Fig.2).Theagesofsedimentlayers,andtime oftheirformation,arequitecloseintheSEGotlandBasinand intheGdańskDeep—thedeepestlayersofformationinthese basinswereestimatedat1838and1858,respectively.The sedimentlayerat21cmdepthintheBornholmBasincomes fromadecidedlylaterperiod—1928,indicatingagreater sedimentationrate.Theseobservationswerecertifiedbythe linearaccumulationrateobtainedfromtheCF:CSmodel.The linear accumulation rate determined in the Gdańsk Deep reached 0.18cmyr1 confirming earlier investigations (Pempkowiak,1991)andwasrelativelyclosetothatinthe Gotland Basin (0.14cmyr1). The corresponding mass

accumulation rates inthese basins were 0.032gcm2yr1 and0.049gcm2yr1.IntheBornholmDeeptheidentified linear accumulation rate and the massaccumulation rate were much higher, 0.31cmyr1 _{and 0.059gcm}2_yr1_, respectively. Fresh water discharge from rivers and the extentofriverinewaterintheseaare thefactors directly influencing the amount of suspended matter in the water column and consequentlyaffect the intensity ofthe sedi-mentationprocess.Ononehand,riverinewaterisasourceof large amounts of organic andmineral material discharged intothesea.Ontheotherhand,thetransportednutrients intensifyprimaryproduction,whichleadstomassive phyto-planktonbloomsunderfavorablemeteorologicalconditions. Takingintoaccountriverinedischargeanditsimpactonthe amountofsuspendedorganicandinorganicmaterialin sea-water,amoreintensiveinfluenceoftheVistularivercouldbe expectedwithintheGdańskDeepareathantheinfluenceof theriverOderintheBornholmDeep,wherenoprolificalgal blooms have been observed and the input of terrestrial materialandpollutionismoderated,e.g.byfiltering proper-tiesoftheSzczecinLagoon.Itisthereforeassumedthatthe largersedimentationrateobservedintheBornholmDeephas to be related to a different structure of the sedimenting organicmatterorspecifichydrologicalconditions.

In order to validate the chronologies determined using 210

Pbdating,theverticaldistributionof137Cswasrelatedto the age of the sediment layers. The distribution of 137Cs activity inthe surfacesedimentlayers reflects, to a large extent,thedistributionof210_Pb

ex(Fig.3).Thehighestvalues wereobservedtheGdańskDeep,whereactivity concentra-tionexceeded200Bqkg1_{inthesurfacelater,whichwasa} littlehigherthenfoundbyZaborskaetal.(2014).The dis-tinctly lower activities of 137_{Cs measured in surface} sedi-ments fromthe BornholmDeep(66.5Bqkg1 intheupper layer)showtheeffectofthefrequentflushesoftheNorthSea (ZalewskaandLipska,2006)salineinflowsinthenearbottom layer,thewaterintheNorthSeahavingmuchlowercontent of137_Cs.137_{CsconcentrationsinsurfacesedimentsoftheSE} Gotland Basin were nearly twice as big, and reached 130.4Bqkg1_.

Thecurvesillustratingchangesof137Csactivityin respec-tive sediment layers and the corresponding years lack unequivocalpeaks (Fig. 3). This maybe attributed to the redistribution ofradiocesiumwithinsedimentcolumn. The redistribution could be caused by two main processes: (i) physicalandbiologicalmixingatornearthesediment-water

6 5 4 3 2 1 0 Cumulative depth [g cm-2_] 1860 1880 1900 1920 1940 1960 1980 2000 2020 Year

Figure2 Correlationbetweenestimatedyearsandcumulative

interfaceand(ii)chemicaldiffusionoradvectionwithinthe porewater.

Theinitialspecificincreasein137Cs activityisobserved since 1950, and this can be related to the beginning of atmospheric nuclear testingstarted in 1945. The increase intestintensitybetween1958and1963couldbeonlyvisible asthecontinuousascensionofintheactivitycurvealongthe vertical profile. 137_{Cs concentrations in sediments show a} constantincreasesince thattime,thoughnote thatinthe entirerange,thelowestactivityratesarecharacteristicof the BornholmDeep sediments. The marked change in the curve slope of the SE Gotland Basin occurred in 1990 as compared to 1980(Fig. 3)and thisis definitely thedirect impact of 137Cs input from the accidental release in the Chernobylpowerplantin1986.Achangeinthecurve devel-opment, thoughlessspectacular, wasalso detectedin the GdańskDeep,andthiswaslinkedtotheyear1994as com-pared to the lower level — dated in 1984. This change indicatesthesamesourceofobservedchanges.

After1995,137_{CsactivityinsedimentsintheSEGotland} Basin became stable as indicated by only an insignificant increasefrom125.5to130.4Bqkg1_{observedin12years.} Similarstabilization of137Cs concentrationsisnoted inthe BornholmDeep,andbetween2001and2006the concentra-tions varied in a narrow range 63.8—66.5Bqkg1. This stabilizationhastobeattributedtothecontinuousdecline oftheisotopeconcentrationsinseawater.Itissolelyinthe areaofGdańskDeepthatanincreasingtendencyofcesium concentrationsisobserved.

Thepresentedtrendsin137Csconcentrationsinsediment coresareacceptabletoverifyfidelityofthe210_Pbdating.

3.2. Heavymetalconcentrations

Sedimentlayers, subjectedto heavymetaldetermination, weredatedusingthe210Pbchronologycharacteristics,taking intoaccounttheshiftrelatedtodifferentdatesofsediment sampling.Inordertoextendtherangeofdating,theresults were extrapolated using a regression model beyond the depthofdating.Thesquarefit(asestablishedforthe corre-lationbetweentheageofsedimentandthecumulativedepth inthedepthrangeofthecoreswheredatingwasperformed) was applied for the results extrapolation. As a result of extrapolation,thedeepestlayers(36—38cm)wereassigned to theyears1625,1751 and1850 intheSEGotlandBasin, GdańskDeepandBornholmDeep,respectively.

Themetalsshowastrongaffinitytotheclayfractionofthe sedimentanditscoatingformation(e.g.organicmatter,iron andmanganese oxides)(Beldowski andPempkowiak, 2003; Pempkowiaketal.,1998,1999;Szeferetal.,1995;Zaborska etal.,2014).Becausesedimentcomposition,andthereforeits grainsize,areliabletovarydependingonthesedimentation area,organicmatterinputfromdifferentsources,andalso dependingonmeteorologicalandhydrologicalconditions,itis necessarytousenormalizationtoeliminatetheeffectofgrain sizeandmineralogyonthefinalresultanditsinterpretation (Acevedo-Figueroaetal.,2006).Thenormalizationprocedure isbasedontheapplicationofaclaymineralindicator,with concentrations ofthe analyzedmetalsthen relatedto this indicator. We have applied Al as the normalizing element (Cheevaporn and San-Diego-McGlone, 1997; Zahra et al.,

2014).Itisaconservativeelementandamajorconstituent oftheclayminerals.Theconcentrationsofheavymetalswere relatedto5%contentofaluminum.Thenormalizationlevelof Alwas based on theobservation thatAl concentrations in sediment cores from the examined areas was relatively uniformand closeto 5%,indicating a homogenicstructure ofsediments.Thegreatestvariabilityinverticaldistributionof AlinsedimentcorewasfoundoutintheGdańskDeep(station P1).Thisbeing theresultofassorted organicmatter trans-portedwiththeriverineinputwithregardtobothcomposition andamount,andtheimpactoftransportedriverineorganic matterismaximalintheGdańskDeepareaascomparedtothe otherbasins.IntheGdańskDeep,thelowestcontentofAlwas determinedinthetwouppermostsedimentlayers(4.72and 4.95%),whilethemaximalcontent(6.34%)wasdeterminedat 32cmdepth.IntheBornholmDeep,Alconcentrationsvaried inaverynarrowrange5.01—5.41%,andthespanof concen-trationsintheSEGotlandBasinwas3.97—4.62%.

Depthprofilesofmetalconcentrationswereconvertedto time-basedprofilesusinga210_{Pb-derivedverticalaccretion} rate(Fig.4).Notsurprisingly,thehighestconcentrationsof allexaminedmetalsweredetectedintheGdańskDeeparea; thepollutantsdepositedbythedirectinputfromtheVistula river(Fig.4).Zincconcentrationinthesurfacelayerreached 245mgkg1 andthiswassimilar to theresultobtainedby

Pempkowiak (1991) (233mgkg1 for the upper layers 2— 4cm)andbyGlasbyetal.(2004)(248mgkg1fortheupper layers2.5—5cm),buthigherthanquoted(148mgkg1)by

Szeferetal.(2009).Inourinvestigation,theleadlevelinthe samelayerwasestimatedat82mgkg1,acomparablefigure to75mgkg1_{obtainedbySzeferetal.(2009).Muchlower} concentrationsweremeasuredinthecaseofcadmiumand mercury,the metalsofstrictlyanthropogenic origin.Their concentrations ranged from 0.17 to 0.05mgkg1, respec-tively, in the deepest sediment core layers to 2.16 and 0.28mgkg1_{intheuppermostpart.SimilarresultsforCd} intheupperlayerwereobtainedinthisregionby Pempko-wiak (1991) — 1.51mgkg1 _{and Glasby et al. (2004) —} 1.7mgkg1. In the Gdańsk Deep, a slight increase of Cd and Hg took place between ca. 1830 and 1940, followed byamorepronouncedchangeinthesemetalsinputintothe marineenvironmentmarkedbyasteepchangeinthecurves' slope.After1980,thecurvesillustrateasubstantial incre-mentleadingtoamaximallevelofmercuryof0.29mgkg1 andofcadmium,1.99mgkg1_{,occurringintheupperlayers.} Zincconcentrationinthesedimentincreasedataslow, nearlyconstantratefrom110mgkg1_{inthedeepestlayerto} 156mgkg1 in1980, fromwhicha steepincreaseto max-imumvalue (246mgkg1_{) reachinginthe upperlayerwas} observed.Leadshowedamuchfaster,andalsocontinuous, accumulation rate in this region, increasing from 7.2 to 43.6mgkg1 up to 1980. Past 1980, the increase in lead concentrationsinthesedimentshowsadecidedlydynamic character. The reason for the more intensive input of Pb shouldbeseeninanoutburstofindustrializationobservedin Polandin 1960 and 1970. None of the metalsanalyzed in sediments from the Gdańsk Deep showed concentration decrease in recent years despite the significant reduction intheiremissionstotheatmosphere.Theemissionof mer-curywasreducedfrom34tyr1in1996to16tyr1in2007, lead— from 960tyr1 _{in 1996 to 553tyr}1 _{in 2007, and} cadmium —from 91tyr1 to 40tyr1 in the sameperiod

(HELCOM, 2009). The maximal riverine input of lead, 210tyr1,wasnotedin1994(HELCOM,2011),althoughthis haddecreasedto180tyr1_{alreadyin1995,andcontinuedto} reach ca. 40tyr1 in 2006. Unfortunately, an increase in riverinedischargesofleadwasobservedin2007,to80tyr1_, causing a reversal of the decreasing trend in the surface sedimentlayer.Theabsenceofsignificantdecreaseinheavy metalconcentrationsinsedimentsfromtheGdańskDeepis probably related directly to the considerable amounts of heavy metals discharged to the sea by the Vistula river. Additionally,anadjournmentoftheresponseofheavymetal concentrationsin surfacesedimentsin relationto changes occurringinthedischargehastobeconsidered,especiallyif thin(2cm)sedimentlayersarestudied.Wellmarkedchanges inconcentrationsofheavymetalsinsurfacesedimentlayer were foundoutin theSE GtolandBasin, wherePb andZn concentrationsshowacleardescentsince1980,andHgsince 1990.HeavymetalconcentrationsinthesedimentfromtheSE GotlandBasinare decidedlylower thanthatintheGdańsk Deep.Particularlylargedifferencesarefoundinthecaseof Cd, Hg and Zn. Cadmium concentrations vary from 0.17mgkg1inthedeepestsedimentlayerto0.51mgkg1 inthesurfacelayer,withasignificantincreasesince1980.A similarpattern,asevidencedbyanincreasesince1980,was notedinHgconcentrations.Mercuryconcentrationsspanned therangefrom0.04to0.12mgkg1_{,andvisibledeclineisseen} inthesurfacelayer,sinceabout1990.Inthecaseofzinc,its

contentincreasedsignificantlyintheSEGotlandBasin sedi-mentsafter1918,andlaterafter1980,reachingamaximumof 188mgkg1_{at4—6cmdepth.Inthisregion,zinc—similarto} lead concentrations, decreased after 1990 to the level of 168mgkg1_{. Lead content showed the lowest gradient} between layers,attaining 43.2mgkg1 at36—38cmdepth andmaximal,72mgkg1,in4—6cmlayerattributedto1990. IntheBornholmDeep,cadmiumandmercury concentra-tions remained practically unchangeable up to 1923, at 0.30 and 0.04mgkg1_{, respectively. Later, the sediment} profilesshow an unvarying increase of both metals up to their maximal levels, Cd — 1.21mgkg1 _{and Hg —} 0.15mgkg1, in surface layers. Cadmium concentration obtainedinthisstudyinsurfacesedimentsoftheBornholm Deep is in very good agreement with the value of 1.20mgkg1 presented by other authors (Szefer et al., 2009).ZnandPbshowadifferent(toCd)patternofchanges intheBornholmDeepsediments.ThePbcurveindicateda considerableshiftaround1890,from24.5mgkg1_inthetwo deepestlayersto34.9mgkg1,andthenextsteepincrease wasnotedafter1950.About1980,Pbconcentrationreached 56mgkg1andstayedalmostunchangedinthenextlayers upto thesurface. Inourstudy, thesurfacesediment con-tained56.6mgPbkg1,alittlelowerlevelthan,85mgkg1, presentedinliterature(Szeferetal.,2009).Inthecaseof zinc,ajumpfrom88mgkg1_to163mgkg1_{wasdefinedto} take place between 1920 and1950. Later on, Zn content 2000 1900 1800 1700 1600 Year 0.0 0.4 0.8 1.2 1.6 2.0 Cd [mg kg -1] / 5% Al 2000 1900 1800 1700 1600 Year 0.00 0.04 0.08 0.12 0.16 0.20 0.24 0.28 Hg [mg kg -1] / 5% Al 2000 1900 1800 1700 1600 Year 0 20 40 60 80 Pb [mg kg -1 ] / 5% Al 2000 1900 1800 1700 1600 Year 80 120 160 200 240 Zn [m g kg -1] / 5% Al

oscillatesaround185mgkg1;theliteraturedatapointouta quitesimilarlevelof188mgkg1(Szeferetal.,2009).

3.3. Enrichmentfactor(EF)

Enrichment factoris widelyapplied to differentiatemetal sources:anthropogenicandnaturalorigin(CarvalhoGomes etal.,2009;Zahraetal.,2014).Enrichmentfactor (EF)is definedastheratioofthegivenmetalconcentration mea-suredintheenvironmentelementtotheconcentrationlevel regarded as the environmental target concentrations. Enhanced valuesof EFindicate theincreased heavymetal concentrations resulting mainly from anthropogenic pres-sure. To illustrate the temporal changes of heavy metal concentrations,enrichmentfactorsEFinparticularsediment layersrelatedto backgroundlevels fromthedeepest layer werecalculatedaccordingtotheformula:

EF¼CML CMB;

whereCML —metalconcentration(normalized to5% Al)in sedimentlayerx,CMB—metalconcentration(normalizedto 5%Al)inbackgroundlayer.

Asanticipated,thehighestEFswereobtainedforallfour heavymetalspeciesinsurfacesedimentsoftheGdańskDeep (Fig.5).InFig.5,theEFvaluesarepresentedascalculatedas aratioofmetalconcentrationineachsedimentlayer—CML to the target concentration of metal — CMT. The highest enrichmentfactorswereobtainedforcadmium;its concen-trations measuredin2009werenearly 13-foldhigherthan the background level. Lead turned out to be the second pollutantwithrespecttoconcentrationincreaseinthe sur-facelayerrelatedtothedeepestlayerwithEF>10.Mercury concentrations increasedover fivetimes,andzincshowed theleastspectacularincrement,withthemaximalEFof2.2. Theweakestchangesinrelationtoreferenceconditions werenotedintheSEGotlandBasin.EFvaluesofPbandZnin thisregionvariedwithinsimilarranges,withamaximalpoint of1.5assignedabout1990.QuitesimilarEFrecords,though atamuchlower levelthanthatin theGdańskDeep,were foundherealsointhecaseofCdwiththemaximumat2.9in thesurfacelayer.Inthecaseofmercury,themaximalEFof 3.0wasfoundaround1980.

In the Bornholm Deep, the build-up of Cd and Hg concentrations in sediment layers were shown to follow approximate patterns as evidenced by the maximalEF of 4.05and4.07,respectively,inthesurfacelayer.Themaximal EFlevelsofzincandleadintheBornholmDeepwere2.27and 2.38,respectively.

Amongthestudiedmarinesedimentationbasins,thearea of GdańskDeep remains under the mostsevere anthropo-genicpressure.TheEFincreasing>1.0,indicatingenhanced inputofheavymetalstothemarineenvironment,datesasfar back as 1828, while the maximal increment gradient was notedafter1979.SEGotlandBasinistheareaoftheweakest anthropogenic impact; the initial ascent of EF values occurred here about 1850, and the subsequent about 1980.TheBornholmDeepEFvaluesofCdandPbarelower thanthatintheGdańskDeepwhilethoseofHgisquiteclose inbothareas.ThefirstpeakingofEFvalues,particularlywell discernedinthecaseofPb,wasnotedhereabout1890.The mostsubstantialincreaseofEFvalues took placebetween

1920and1980,withCdandHgconcentrationscontinuingto increasewhilePbcontentstartedtostabilize.

3.4. Environmentalstatusassessment regarding marinesedimentpollutionwithheavymetals

Environmental quality assessment requires application of appropriate indicators, which would unequivocally reflect

2020 1980 1940 1900 1860 1820 1780 1740 Year 0 1 2 3 4 5 6 7 8 9 Enrichment factor 2000 1960 1920 1880 1840 Year 0.8 1.2 1.6 2.0 2.4 2.8 3.2 3.6 Enrichment factor 2000 1900 1800 1700 1600 Year 0.8 1.2 1.6 2.0 2.4 Enrichment factor

Figure5 EnrichmentfactorsestimatedforCd,Hg,PbandZnin

thepresentsituation.Regardlessoftheappliedindicator,the key element in environmental quality assessments is the definitionofreferenceconditionstowhichthecontemporary valuesarerelated.Thereforeitisveryimportanttoestablish appropriatetargetconcentrations;thistargetlevelcouldbe determinedasaninnocuousconcentration,notcausingany biologicaleffectsinmarineorganisms.Wehaveassumedthat the metalconcentrations in the deepest sediment layers, correspondingto theabsenceof or verylimited anthropo-genicpressure, fulfill the definition of target values. The mean concentration of each metal in the three deepest sedimentlayers (22—24cm,29—31cmand 36—38cm)was assumedas themethod oftarget values setting for heavy metalpollutionassessment inthesouthernBalticSea.The following target concentrations (CT) were obtained: Zn

—110mgkg1, Pb— 30mgkg1,Cd — 0.3mgkg1and Hg

—0.05mgkg1_{. The presented values are in considerable} agreementwiththeaveragemetalconcentrationsinshale, thesereaching accordingto TurekianandWedepohl(1961)

andZahraetal.(2014):Zn—95mgkg1_,Pb—20mgkg1 andCd—0.3mgkg1.

The determinedtargetvalues wereappliedtocalculate the relevant indices further used in environmental status assessment(CarvalhoGomesetal.,2009;Zahraetal.,2014):

(1)Enrichmentfactor(EF) EF¼CM0_CMT;

whereCM0—heavymetalconcentrationinthesurface sedimentlayer,CMT—targetconcentrationofthemetal. The enrichment factors (EF),besides supplyingthe basicinformationonthecontributionofanthropogenic pollutants,arealsousedinenvironmentalstatus assess-mentsasputforwardintheclassificationpresentedin

Table2(BirchandOlmos,2008;Zahraetal.,2014). (2)Geoaccumulationindex(Igeo)

Igeo¼log2CM0 1:5CMT :

Igeoisacoefficient,proposedbyMüller(1979),andquite widely applied in sediment studies (Carvalho Gomes etal.,2009;Zahraetal.,2014),includingassessments ofsedimentpollutionwithheavymetals(Zahraetal., 2014).

(3)Contaminationfactor(CF)

The method of CF calculation is identical to EF calculation.ThisfactorwasappliedintheInitial Assess-ment of the Polish marineareas within MSFD (Anon., 2008)implementationprocess.

TheCFfactorwasappliedtoassessthestatusofmarine environmentbasedon5-classscale,derivedfortheEUWater Framework Directive (FWD) (Anon., 2000) purposes of the marine transitional andcoastal waters' statusassessments (Table2).However,theMSFDrequirestheclassificationtobe

Table2 Sedimentqualityclassificationsbasedonenrichmentfactor(EF),geoaccumulationindex(Igeo)andcontaminationfactor (CF).

EFclassificationa Igeoclassificationa CFclassification

EFranges EF

classes Status description

Igeoranges Igeo

classes Status description CFranges CF classes Status description according 5-classes assessment Status according MSFD

EF<1 I Noenrichment Igeo=0 0 Unpolluted 0<CF<0.5 5 Verygood

GES(Good Environmental Status) EF<3 II Minor enrichment 0<Igeo<1 1 Unpolluted to moderately polluted 0.5CF<1 4 Good 3_<EF_<5 III Moderate enrichment 1_<Igeo<2 2 Moderately polluted 1CF_<5 3 Moderate Sub-GES 5<EF<10 IV Moderately severe enrichment 2<Igeo<3 3 Moderately tohighly polluted 5CF<10 2 Poor 10_<EF_<25 V Severe enrichment 3_<Igeo<4 4 Highly polluted CF10 1 Bad 25<EF<50 VI Extremely severe enrichment 4<Igeo<5 5 Highlyto veryhighly polluted — — — —

— — — 5<Igeo<6 >5 Veryhighly

polluted

— — — —

done in two categories only: Good Environmental Status (GES)andsub-GES(Table2).

TheCFcoefficientshavebeencalculatedofallanalyzed metalconcentrationsinsurfacesedimentlayers(Table3)as the current environmental status was to be assessed. The obtainedCFvalueswereusedtoclassifytheenvironmental statusinthethreesouthernBalticSeaareaswithregardto heavymetalpollution.InthecaseofZnandPbthestatusofall three areas assessed on thebasis ofIgeoand EF has to be consideredasunpollutedtomoderatelypollutedasonlyminor enrichmentregardingthesemetalsoccurs(Table3).CdandHg presentquitedifferentcase,asthevaluesoftheirIgeoindicate a moderatelyto highlypolluted status ofsediments in the GdańskDeep,andmoderatelysevereenrichmentisobserved accordingtotheEF'svalues.IntheBornholmDeep,thereis moderateenrichmentasregardCdandminorenrichmentin thecaseofHg.TheSEGotlandBasinhastobeconsideredas unpollutedtomoderatelypollutedwithCdandHg.

TakingintoaccounttheCFfactor,thestatusofallareas classifiedwithrespecttoallanalyzedmetalswasmoderate, inthe5-classscaleandassub-GESintheMSFDclassification system.

Toassesstheaggregatedimpactoftheanalyzedmetalsan averageofallindicatorswascalculated(Table2), character-izingtheassessedarea.Theobtainedresultspointedoutthe Gdańsk Deep to be moderately polluted, with moderate enrichmentinheavymetals,whiletheBornholmDeepand theSEGotlandDeepturnedouttobeunpollutedto moder-ately polluted with minor enrichment of heavy metals in sediments.

Theobtainedclassificationresults,basedontheapplied indicators,fallingoodagreement,particularlyregardingthe EF and Igeo. Wider ranges of EF and Igeo values make the classificationschemeflexible,whilethenarrowrangeofCF valuesmakestheclassificationmorerigorous.

Themostrestrictiveis,however,theuseof2-classsystem recommendedbyMSFD,asthesub-GESresultofclassification imposesanobligationofcountermeasurestobeundertaken on land,though simultaneously themagnitude of the pro-blemcannotbeassessedproperly.

4.

Conclusions

The area of Gdańsk Deep remains under the most severe anthropogenic pressure among the studied sedimentation basins. Sediments from thisregion were characterized by the highest heavy metal concentrations, except for Pb — higherconcentrationofleadwasfoundinSEGotlandBasin before 1880. The substantial, saltatory change in metal concentrations and theresulting EF values were observed intheGdańskDeepafter1979.Themaximalmetal concen-trationsherereached 232mgkg1 —Zn,77mgkg1 —Pb, 2.04mgkg1 _{—Cdand0.27mgkg}1_{—Hg;thisresultedin} enrichmentfactorvalues (related totheconcentrations in thedeepestlayer)of13inthecaseofCd,10—Pband5—Hg. ThispollutionofGdańskDeepsedimentswithheavymetalsis directlylinkedtotheirinputviathefreshwaterdischargeof theVistulaRiver.

The SE Gotland Basin is the area showing the weakest anthropogenicpressure,thisbeingprovedbyboththeactual heavy metal concentrations and the dynamics of their changes.ThesubstantialincreaseinCdandHgaccumulation insedimentstookplaceafter1980,andthiswasevenmore dynamicthanin theGdańskDeep. ThemaximalEF values observedherereached1.5inthecaseofZnandPbandabout 3inthecaseofCdandHg.

Metal concentrations in the Bornholm Deep sediments assumed intermediate levels; simultaneously the greatest differentiation was seen after 1920, when a significant increaseinheavymetalinputoccurredinthisregion.

The determined target concentrations (CT): Zn

—110mgkg1, Pb— 30mgkg1,Cd — 0.3mgkg1 andHg

—0.05mgkg1areconsistentwiththemeanconcentrations specificofaverageconcentrationsinshale.

Onthebasisofassessmenton geoaccumulationindex— Igeo,enrichment factor—EFandcontaminationindex,the areaoftheGdańskDeepisconsideredmoderatelypolluted with moderate enrichment of sediments in heavy metals, whiletheareasofBornholmDeepandSEGotlandBasinare unpollutedtomoderatelypollutedwithminorenrichmentof sedimentswithheavymetals.Inthecaseofassessmentbased

Table3 Valuesofenrichmentfactor(EF),geoaccumulationindex(Igeo)andcontaminationfactor(CF)determinedforeachmetal

andaggregatedsedimentstatusassessment.

Zn Pb Cd Hg Sedimentstatusassessment

EF/CF Igeo EF/CF Igeo EF

(CF)

Igeo EF/CF Igeo MeanEF MeanIgeo Mean

CF/5-classes assessment Mean CFMSFD assessment GdańskDeep (P1) 2.2 0.6 2.7 0.9 7.2 2.3 5.7 1.9 4.5 Moderate enrichment 1.4 Moderately polluted 4.5 Moderate 4.5 SubGES Bornholm Deep(P5) 1.7 0.2 1.9 0.3 4.1 1.4 2.9 1.0 2.6 Minor enrichment 0.7 Unpollutedto moderately polluted 2.6 Moderate 2.6 SubGES SEGotland Basin (P140) 1.5 0.03 2.2 0.6 1.7 0.2 2.3 0.6 1.9 Minor enrichment 0.4 Unpolluted tomoderately polluted 1.9 Moderate 1.9 SubGES

on CFfactor, all areaswereclassifiedas havingmoderate statusorsub-GESinthe2-classassessment.

The obtainedresultspointtodifferencesin characteris-ticsanddynamicsofsedimentformationinthebasinslocated intheeasternpartofthePolishsectorofthesouthernBaltic Sea— GdańskDeep and SEGotland Basin and thatin the westernpart—theBornholmDeep.Theperiodsofsediment formationintheGdańskDeepandSEGotlandBasinarevery similar; the deepest layers were respectively dated in 1838 and 1858, while the deepest sediment layers from the Bornholm Deep denote a much later period, around 1928,pointingto afaster sedimentationratein thisarea. The determined linear sedimentation rates in the Gdańsk Deep (0.18cmyr1) and in the SE Gotland Basin (0.14cmyr1_{)are quiteclose,andthecorrespondingmass} accumulation rates reached: 0.032gcm2yr1 and 0.049gcm2_yr1_{. In the Bornholm Deep higher values of} bothlinearsedimentation(0.31cmyr1)andmass accumu-lation(0.059gcm2yr1)ratesweredetermined.

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